Structural assessment and catalytic consequences of the oxygen coordination environment in grafted Ti-calixarenes

Article abstract of DOI:10.1021/ja065830c

Calixarene-Ti complexes were grafted onto SiO₂ (0.18-0.24 Ti nm^-2) to form isolated and accessible Ti centers persistently coordinated to multidentate calixarene ligands. Grafted Ti-tert-butylcalix[4] arenes gave Ti K-edge absorption spectra with pre-edge features at 4968.6-4968.9 eV, independently of Ti surface density and of their use in epoxidation catalysis. The structure and reactivity of grafted Ti-calix[4]arenes were weakly dependent on thermal treatment below 573 K, and the relative epoxidation rates of trans- and cis-alkenes showed that calixarene ligands did not restrict access to Ti centers more than corresponding calcined Ti-SiO₂ materials. For all materials, ¹³C NMR and UV-visible spectroscopies confirmed the presence of Ti-O-Si connectivity and identical ligand-to-metal transitions. Grafted Ti-homooxacalix[3]arene complexes, however, gave weaker pre-edge features at higher energies (～4969.5 eV), consistent with greater Ti 3d occupancy and coordination numbers greater than four, and 20-fold lower cyclohexene epoxidation rate constants (per Ti) than on calix[4]arene-based materials. These different rates and near-edge spectra result from aldehyde formation caused by unimolecular cleavage of ether linkages in homooxacalix[3]arene ligands during grafting, leading to higher coordination and electron density at Ti centers. Materials based on tert-butylcalix[4]arene and homooxacalix[3]arenes led to similar epoxidation rates and near-edge spectra after calcination, consistent with the conversion of both materials to isolated Ti centers with identical structure. These materials provide a systematic approach for relating oxidation reactivity to Ti 3d occupancy, a descriptor of Lewis acid strength, and Ti coordination, because they provide Ti centers with varying electron density and coordination, but maintain accessible active centers with uniform structure and unrestricted access to reactants.

Full text of DOI:10.1021/ja065830c

Published on Web 01/17/2007
Structural Assessment and Catalytic Consequences of the
Oxygen Coordination Environment in Grafted Ti-Calixarenes
Justin M. Notestein,† Leandro R. Andrini,‡ Vitaly I. Kalchenko,§ Felix G. Requejo,*,‡
Alexander Katz,*,† and Enrique Iglesia*,†
Contribution from the Department of Chemical Engineering, UniVersity of California at
Berkeley, Berkeley, California 94720, Departamento de F´ısica, Facultad de Ciencias Exactas,
UniVersidad Nacional de La Plata and IFLP-INIFTA (CONICET), 1900 La Plata, Argentina,
Institute of Organic Chemistry, National Academy of Sciences of Ukraine,
Murmanskaya Str., 5, 02094, KyiV-94, Ukraine
Received August 10, 2006; E-mail: requejo@fisica.unlp.edu.ar; askatz@berkeley.edu; iglesia@cchem.berkeley.edu
Abstract: Calixarene-Ti complexes were grafted onto SiO2 (0.18-0.24 Ti nm-2) to form isolated and
accessible Ti centers persistently coordinated to multidentate calixarene ligands. Grafted Ti-tert-butylcalix-
[4]arenes gave Ti K-edge absorption spectra with pre-edge features at 4968.6-4968.9 eV, independently
of Ti surface density and of their use in epoxidation catalysis. The structure and reactivity of grafted Ti-
calix[4]arenes were weakly dependent on thermal treatment below 573 K, and the relative epoxidation
rates of trans- and cis-alkenes showed that calixarene ligands did not restrict access to Ti centers more
than corresponding calcined Ti-SiO2 materials. For all materials, 13C NMR and UV-visible spectroscopies
confirmed the presence of Ti-O-Si connectivity and identical ligand-to-metal transitions. Grafted
Ti-homooxacalix[3]arene complexes, however, gave weaker pre-edge features at higher energies (∼4969.5
eV), consistent with greater Ti 3d occupancy and coordination numbers greater than four, and 20-fold
lower cyclohexene epoxidation rate constants (per Ti) than on calix[4]arene-based materials. These different
rates and near-edge spectra result from aldehyde formation caused by unimolecular cleavage of ether
linkages in homooxacalix[3]arene ligands during grafting, leading to higher coordination and electron density
at Ti centers. Materials based on tert-butylcalix[4]arene and homooxacalix[3]arenes led to similar epoxidation
rates and near-edge spectra after calcination, consistent with the conversion of both materials to isolated
Ti centers with identical structure. These materials provide a systematic approach for relating oxidation
reactivity to Ti 3d occupancy, a descriptor of Lewis acid strength, and Ti coordination, because they provide
Ti centers with varying electron density and coordination, but maintain accessible active centers with uniform
structure and unrestricted access to reactants.
and infrared and Raman10-13 spectroscopies can probe various
structural and electronic features of isolated Ti sites. Ti K-edge
Introduction
Highly dispersed Ti centers dispersed onto or within SiO2
frameworks are active catalysts for selective oxidations and other
molecular rearrangements catalyzed by Lewis acids,1-4 because
the resulting 4-coordinate Ti centers are electron deficient and
can expand their coordination to bind oxidants or substrates and
to activate oxidants toward attack by electron-rich substrates.
These Ti centers have been widely studied since the first reports
of TS-15 and MCM41-grafted Cp2TiCl26 materials as prototypi-
cal Ti-SiO2 catalysts. Previous studies have tried to develop
routes for the synthesis of isolated Ti centers. UV-visible7-9
X-ray absorption spectroscopy remains, however, the method
of choice to probe the local coordination of dispersed Ti
structures.10,14
We report here the use of X-ray absorption methods on
surface-grafted Ti-SiO2 catalysts based on calixarene-Ti
complexes 2a and 2b (Scheme 1), which catalyze epoxidation
reactions with turnover rates independent of Ti surface density.
Such properties are accepted hallmarks of uniform single-site
catalysts15 and evidence for isolated 4-coordinate Ti centers.
(7) Soult, A. S.; Poore´, D. D.; Mayo, E. I.; Stiegman, A. E. J. Phys. Chem. B
2001, 105, 2687-2693.
† University of California at Berkeley.
‡ Universidad Nacional de La Plata and IFLP-INIFTA (CONICET).
§ National Academy of Sciences of Ukraine.
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J. AM. CHEM. SOC. 2007, 129, 1122-1131
10.1021/ja065830c CCC: $37.00 © 2007 American Chemical Society

Oxygen Coordination in Grafted Ti−Calixarenes
A R T I C L E S
Scheme 1a,b
a Reactions and conditions: (i) Contact 1a or 1c with 1 equiv Ti(OiPr)4 in toluene, 48 h; (ii) reflux 2a with 1 equiv triphenylsilanol in toluene, 24 h; (iii)
reflux 2a or 2c with partially dehydroxylated silica gel in toluene, 24 h; (iv) reflux 1b with 1 equiv TiCl4 in toluene, 24 h; (v) reflux 2b with partially
dehydroxylated silica gel and 10 equiv 2,6-di-tert-butylpyridine in toluene, 24 h. b Bracketed structure has not been isolated.
Single-site behavior, however, requires neither isolated Ti nor
tetrahedral coordination; a notable example is the construction
of identical 5-coordinate Ti-O-Ti dimers on SiO2 under several
grafting conditions.16 Here, we use Ti K-edge X-ray absorption
near-edge spectroscopy (XANES) to determine the Ti coordina-
tion number and the transition probability to unoccupied Ti 3d
electronic states in these materials as a function of treatment
temperature, Ti surface density, Ti precursor (TiCl4 or TiOiPr4),
and catalyst use in epoxidation reactions. Calixarene-Ti and
structurally related complexes have not been previously exam-
ined by XANES; in view of this, we prepared and characterized
a soluble model compound 4a as a standard material that
contains all relevant calixarene-Ti and Ti-siloxy connectivities
likely to exist in grafted complexes.
turn to other calixarene macrocycles to demonstrate the rel-
evance of these ligands to the Ti coordination in the resting
state during catalytic turnovers. The substitution of some of the
calixarene phenolic OH groups with methoxy groups can be
used to vary the number of linkages between Ti centers and
calixarene ligands,17-22 but the catalytic consequences of these
changes in coordination remain unexplored. Here, we show that
grafting calix[4]arene-Ti complexes 2a and 2b leads to isolated
4-coordinate Ti, while homooxacalix[3]arene-Ti complexes 2c
(Scheme 1) also form isolated sites, but with Ti coordination
greater than four. Grafting 2c leads to lower epoxidation rates
than calixarene complexes with 4-coordinate Ti centers, dem-
onstrating the catalytic relevance of Ti-calixarene coordination
and also that grafting of calixarene-Ti complexes leads to
XANES is used here to measure the coordination number of
surface-grafted Ti species coordinated to calixarene ligands,
some of which may not persist during catalysis. We therefore
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J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1123

A R T I C L E S
Notestein et al.
chambers and setting the first inflection point at the known absorption
edge energy of Ti0 (4966 eV).23 Energies below 4960 eV were fit to a
Victoreen function and used to subtract background contributions
throughout the entire energy range. Absorption intensities were
normalized by their average value between 5050 and 5200 eV.32
functional materials different from ligand-free Ti centers formed
by oxidative treatments of organometallic precursors. The
synthetic organic chemistry of macrocycles with varying
coordination number and the subsequent grafting of their Ti
complexes provide a route to develop new catalysts and to test
the catalytic relevance of coordinative unsaturation in hetero-
geneous oxidation catalysis.
Samples were treated at the specified temperature to remove adsorbed
water and sealed into the sample holder within a controlled atmosphere
box filled with dry Ar before measuring spectra. Untreated and liquid
samples were sealed into the sample holder in ambient air. Spectra for
soluble model compounds were measured using 0.1 M solutions in
anhydrous toluene. Previous experimental and theoretical studies have
identified three features in the Ti K-pre-edge region for anatase and
other Ti oxides,33-36 but higher energy resolution spectra have shown
instead four distinct pre-edge features.37-39 After background subtraction
and normalization, the pre-edge region was fitted to four Gaussian
peaks, labeled in ascending energy A1, A2, A3, and B, as in previous
studies,36,40 using WinXAS.41
For Ti centers, an increase in the number of 3d electrons,
determined by the number and identity of coordinating ligands,
decreases Lewis acidity. In Ti K-edge XANES, the intensity of
the pre-edge feature provides a direct measure of available Ti
3d orbitals.23 The presence of intense XANES pre-edge features
has been empirically related to the presence of 4-coordinate
species and to highly active oxidation catalysts,3,24-26 but a more
complete analysis of the structural requirements for Ti-based
Lewis-acid catalysis and of the fidelity of pre-edge features as
descriptors of catalytic reactivity have remained elusive, at least
in part because of the dearth of materials with uniform structure,
accessible Ti centers, and coordination numbers larger than four.
Sol-gel TiO2-SiO2 mixed oxides27 and amorphous compounds
containing Na2O28 have shown average Ti coordination numbers
of five, but invariably contain a distribution of TiO2 cluster sizes
and Ti coordination numbers. In addition, the accessibility of
Ti centers is incomplete and often uncertain. To our knowledge,
JDF-L129 is the only synthetic material with only 5-coordinate
Ti; it was reported to be active for phenol oxidation after HCl/
H2O2 treatment, but rate data were not reported. Grafting
4-coordinate Ti centers on Sn- or Ge-modified SiO230 alters the
reactivity of Ti centers but without significant changes in near-
edge spectra. The adsorption of amino alcohols or diamines onto
preexisting Ti-SiO231 markedly decreases epoxidation rates, but
XANES data were not reported and the uniform nature of Ti
centers remains unproven. The surface-grafted calixarene-Ti
complexes reported here are ideally suited to probe the relation
between near-edge features and turnover rates for various Ti
environments, because of the common chemistries of the ligands
involved and the uniform and accessible nature of the active Ti
centers.
Catalyst and model compound syntheses are illustrated in Scheme
1. All syntheses were performed using standard Schlenk line methods.
Toluene was distilled in the presence of CaH2 before use. Compound
1a was synthesized from tert-butylcalix[4]arene (Aldrich, 95%) using
accepted methods.42 Compound 1b was purchased from commercial
sources (Acros, 99%). Compound 1c was synthesized through an acid-
catalyzed route.43 Compounds 2a19 and 2c44 were synthesized by adding
1 equiv Ti(OiPr)4 (Aldrich 99.999%) to a 0.1 M toluene solution of 1a
or 1c and stirring for 48 h. These reactions are known to proceed with
nearly quantitative yields. Proton NMR resonances and mass spec-
trometry of these samples are consistent with published values.
Compound 4a was synthesized by adding a solution of 2a to 1 equiv
triphenylsilanol (Aldrich 98%) and refluxing for 24 h under flowing
N2. Its structure was verified by 1H, 13C, and 29Si NMR spectroscopies,
elemental analysis, and mass spectrometry, as described in the
Supplemental Information. Catalysts 3a-l, 3a, and 3c were prepared
by adding, respectively, 0.1 mmol 2a, 0.25 mmol 2a, or 0.25 mmol 2c
per gram of SiO2 (0.6 nm pore diameter, 250-500 µm particle diameter,
partially dehydroxylated under dynamic vacuum at 773 K for 24 h,
Selecto) in sufficient toluene to suspend the solids with magnetic
stirring. The suspension was refluxed for 24 h and sparged with N2 at
388 K until dry. The solids were washed with boiling toluene until
calixarene complexes were no longer detected in the eluted liquids,
and then dried under dynamic vacuum for 4 h at ambient temperature
and for 4 h at 393 K.
Catalysts were stored in ambient conditions until use. The numbers
listed after the dash in notation used herein indicate the treatment
temperature in Ar before measuring Ti K-edge XANES spectra or in
dynamic vacuum before measuring epoxidation rates or solid-state NMR
spectra. Calixarene ligands were completely removed via treatment in
flowing N2/O2 at 823 K to produce calcined materials 3a-823 and 3c-
Experimental Methods
Ti K-XANES spectra were acquired using a fluorescence detector
and a Si (111) monochromator at the D04B-XAS beamline of the LNLS
(Laboratorio Nacional do Luz S´ıncrotron, Campinas, Brazil). Harmonic
beam components were less than 1%, the incident beam energy was
1.37 MeV, and the resolution was 0.8 eV. The beam intensity was
measured using ionization chambers filled with He at ambient tem-
perature and pressure. Photon energies were calibrated in transmission
mode by using a Ti foil placed between the second and third ionization
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1124 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Oxygen Coordination in Grafted Ti−Calixarenes
A R T I C L E S
823. These materials were treated at 393 K in Ar or dynamic vacuum
immediately prior to XANES spectra acquisition or measuring epoxi-
dation rates, respectively. Compound 2b and catalyst 3b were prepared
as described previously.15 This preparation method includes treatment
under dynamic vacuum for 1 h at 523 K immediately after synthesis.
The material was stored in ambient air.
Materials 3a-u and 3c-u were used in epoxidation reactions, as
described above, except with 300 mg catalyst, 50 mL n-octane, 0.81
mL cyclohexene, and 0.36 mL TBHP, which gave ∼11 turnovers at
100% conversion. After 1 h the catalysts were hot filtered, washed
with hot octane, and dried at 393 K for 1 h in dynamic vacuum. XANES
and 13C CP/MAS NMR spectra were measured to detect any changes
in Ti coordination environment after catalysis.
Thermogravimetric measurements were conducted (TGA 2950) in
flowing dry N2 or synthetic air (1.5 cm3 s-1 N2 or 0.5 cm3 s-1 O2 +
1.5 cm3 s-1 N2 as boil-off from liquid) by heating samples from ambient
temperature to 1073 K at 0.083 K s-1 in a Pt pan. These methods were
used to measure the number of calixarenes in each sample by assuming
that the mass is lost by combustion of all organic fragments in
calixarenes-Ti complexes 2a, 2b, or 2c, with molecular weights of
654, 611, or 568, respectively. The presence of isopropoxide groups
chemisorbed on the surface SiOH groups for 3a and 3c was confirmed
by 13C CP/MAS NMR and included in the calculation. This method
predicts carbon contents in agreement with the 48:1, 45:1, and 39:1
C:Ti atom ratios expected from complexes 2a, 2b, and 2c, respectively.
Ti contents measured by Quantitative Technologies, Inc. using induc-
tively coupled plasma mass spectrometry were 0.83, 0.48, and 0.67 wt
% Ti for materials 3a, 3a-l and 3c, respectively, and agree to within
5% of the surface densities calculated by TGA.
Results and Discussion
Physicochemical Comparison of Materials 3a, 3b, and 3c.
Materials 3a, 3b, and 3c were prepared with an excess of
calixarene-Ti precursors in the contacting solution, but they
contain similar calixarene-Ti surface densities (0.18-0.24 Ti
nm-2), which correspond to at most 30% of the density of SiOH
groups on the SiO2 used.46 This value resembles that for the
maximum achievable density for random irreversible deposi-
tion47 of non-interacting calixarenes ∼1.5 nm in diameter. As
shown for other calixarene-Ti materials,15 this saturation
behavior indicates that the Ti content is limited by the cross-
sectional area of the calixarene ligand, whose steric bulk and
tripodal chelation prevents Ti-O-Ti connectivity.
UV-visible spectra were measured at ambient conditions and
temperature using a Varian Cary 400 Bio UV-visible spectrophotom-
eter with a Harrick Praying Mantis accessory for diffuse-reflectance
measurements of powders. Compressed poly(tetrafluoroethylene) was
used as a perfect reflector standard to subtract background spectra and
calculate pseudo-absorbances using the Kubelka-Munk formalism.45
Solid-state 1H MAS and 13C CP/MAS NMR spectra were collected at
the California Institute of Technology solid-state NMR facility using
a Bruker DSX500 spectrometer operating at 500 MHz. Infrared spectra
were measured using a Nicolet NEXUS 670 infrared spectrometer
equipped with a Hg-Cd-Te (MCT) detector by averaging 5000 scans
using samples diluted with KBr and held within a quartz vacuum cell
with NaCl windows. Spectra were measured with 2 cm-1 resolution in
the 4000 cm-1 - 400 cm-1 frequency region. Samples were heated in
vacuum (<0.1 Torr) to 393 K (0.05 K s-1) for 1 h before acquiring
infrared spectra.
Cyclohexene epoxidation rates and selectivities were measured as
follows: 30 mg catalyst and ∼200 mg of zeolite A (previously dried
at 573 K for 12 h in dynamic vacuum) were added to a 25 mL round-
bottom flask and heated to the prescribed treatment temperature for 1
h in dynamic vacuum. The flask was filled under Ar and 20 mL
anhydrous n-octane (Aldrich, 99.8%, passed over a SiO2/Al2O3 column,
and freshly distilled off Na metal) and 0.3 mL cyclohexene (Aldrich,
99%, passed over Al2O3 column, freshly distilled off CaH2) were added
to the reactor. The reactor was sealed and brought to 333 K. A solution
of tert-butyl hydroperoxide (TBHP) in nonane (0.125 mL, 4.8 M
solution in nonane, Aldrich, dried over 4A sieves) was added to initiate
the reaction. Reaction rates and selectivities were measured by removing
liquid aliquots and introducing them into a gas chromatograph (Agilent
6890, HP-1 methylsilicone capillary column).
The proposed structures of these materials were confirmed
by solid-state NMR and diffuse reflectance UV-visible spec-
troscopy. 13C CP/MAS NMR spectra of 3a and 3c (Figure 1)
give resonances at 67 and 22 ppm, indicative of isopropoxides
adsorbed on silica (Si-OiPr),16,48 without the sharp features at
∼78 ppm16 expected if Ti- OiPr connectivity persisted after
grafting of Ti isopropoxide precursors. The other resonances
for 3a are similar to those for 2a in solution19 and to those for
3b.15 The solid-state NMR spectrum of 3a is not visibly
influenced, in chemical shift or intensity, by thermal treatment
(up to 573 K) or by the use of the material in cyclohexene
epoxidation.15 The 13C CP/MAS NMR spectrum of 3c shows
the aryl and tert-butyl resonances also present in the solution
spectrum of 2c,44 together with weak resonances for benzyl ether
bridging groups (resonance 4). The spectrum for 3c also shows
a new resonance at ∼190 ppm, which suggests the presence of
carbonyl groups (resonance 8). This conclusion is confirmed
by the appearance of bands in the carbonyl region of the infrared
spectra at 1560, 1640, and 1680 cm-1 (Figure S1) for this
sample, which are absent for 1c. These carbonyl signatures
appear to reflect thermal and acid-catalyzed rearrangements of
benzyl ether bridges49 in the homooxacalix[3]arene ligand 1c
during grafting and thermal treatment to form an aldehyde and
a methyl group. Grafted calix[4]arene material 3a contains no
features in the range of 1500-2000 cm-1. Evidence of an
aldehyde was obtained by extracting calixarene active sites from
3c into CDCl3 via treatment with trimethylsilyl chloride and
subsequently observing a discrete resonance at 9.9 ppm in the
solution 1H NMR spectrum of the extracted sites.50 The resulting
carbonyl is likely to coordinate to the neighboring Ti center;
indeed, salicylaldehyde-Ti complexes with simultaneous co-
ordination of phenolate and aldehyde groups to a single Ti atom
Calixarene-Ti materials are highly selective, heterogeneous catalysts
for epoxide synthesis and epoxidation rates obey the following rate
equation at these reactant and catalyst concentrations:15
d[epoxide]
rate )
) k1[Ti][alkene][TBHP]
(1)
dt
(46) Zhuravlev, L. T. Colloids Surf. A 2000, 173, 1-38.
where [Ti] is the total concentration of Ti centers in the reactor. All
concentrations are in moles per L, resulting in a k1 with units of M-2
s-1. This rate constant (k1, per Ti) was independent of Ti surface density,
consistent with the uniform structure and single-site character of Ti
centers in these samples.15
(47) Fadeev, A. Y.; Lisichkin, G. V. Stud. Surf. Sci. Catal. 1996, 99, 191-212.
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(50) 3c (56 mg of material) (0.18 mmol Ti g-1) was treated at 393 K under
<50 mTorr vacuum for 30 min. Then 0.01 mmol trimethylsilyl chloride
(20 equiv per Ti) in 1 mL of dichloromethane was added at room
temperature, and the solution was stirred for 30 min, decanted, and filtered
under inert atmosphere. The filtrate was concentrated under vacuum and
analyzed via 1H NMR spectroscopy in CDCl3.
(45) Delgass, W. N.; Haller, G. L.; Kellerman, R.; Lunsford, J. H. Spectroscopy
in Heterogeneous Catalysis; Academic Press: New York, 1979.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1125

A R T I C L E S
Notestein et al.
Figure 1. Solid-state 13C CP/MAS NMR spectra of 3a and 3c. Resonances are labeled as in the accompanying structures. (*) indicates spinning sideband
and (+) indicates resonances due to surface isopropoxy groups, the latter of which is emblematic of covalent surface grafting of the Ti complex. The
resonance at ∼190 ppm is indicative of a carbonyl group, whose intensity is extremely sensitive to catalyst pretreatment, presumably due to the long
relaxation time of this group.
are known51 and give carbonyl bands at ∼1600 cm-1 shifted to
lower frequencies from those in free salicyaldehyde ligand.52
For 3c, the carbonyl features in the NMR decreased in intensity,
while the infrared band at 1640 cm-1 increased in intensity,
after exposure to tert-butyl hydroperoxide, suggesting conver-
sion of one carbonyl to another. Sequential aldehyde oxidation
to an arenecarboxylic acid under reaction conditions cannot be
ruled out.53,54
The calixarene-Ti ligand-to-metal charge transfer (LMCT)
band responsible for the absorption edge55,56 in UV-visible
spectra (Figure S2) confirmed the calixarene-Ti connectivity
shown in Scheme 1. The edge energies occur at 2.20 and 2.22
( 0.04 eV for 3a and 3c, respectively. A similar edge energy
(2.18 eV) was previously observed for 3b,15 indicating that the
proposed Ti-phenolate connectivity is present and similar in
these three samples. Soluble compound 4a (0.1 mM in
chloroform) gives an edge energy of 2.37 eV; a small red shift
in the LMCT band is expected57 as the species move from the
Figure 2. Ti K-XANES spectra of reference Ti-compounds at ambient
temperature in air: (a) 4-fold coordinate [4]Ti in Ba2TiO4; (b) [5]Ti in
low dielectric chloroform solution to a hydroxylated SiO2
Fresnoite; (c) [5]Ti in nanostructured anatase (average crystal size <6 nm),59
surface with a higher dielectric constant.58 Comparisons between
solution and solid-state NMR and the insensitivity of the LMCT
(d) [6]Ti in rutile, and (e) [6]Ti in anatase.
that 3b consists of uniform Ti centers on a SiO2 surface.15 The
similarity of the physicochemical characterization data for 3a
energy to Ti surface coverage were previously used to show
and 3c to those for 3b suggests that the former also consist of
a single type of catalytic site, and that, with the exception of
the presence of isopropoxide groups co-grafted on SiO2, 3a is
indistinguishable from 3b.
Coordination Environment of Calix[4]arene Complexes.
Figure 2 shows Ti K-edge XANES spectra for Ti oxides and
Ti silicates with known structures. Near-edge spectra for all
samples showed a predominant pre-edge feature at ∼4969 eV
(Figure 3), as in 4-coordinate Ti ([4]Ti) in Ba2TiO2 and
5-coordinate Ti ([5]Ti) in the mineral fresnoite. Ti K-edge
XANES spectra for 3a are also reported in Figure 3 as a function
(51) Pennington, D. A.; Clegg, W.; Coles, S. J.; Harrington, R. W.; Hursthouse,
M. B.; Hughes, D. L.; Light, M. E.; Schormann, M.; Bochmann, M.;
Lancaster, S. J. Dalton Trans. 2005, 561-571.
(52) Chaudhry, S. C.; Bhatt, S. S.; Sharma, N. Ind. J. Chem. 1996, 35A, 520.
(53) Subrahmanyam, C.; Louis, B.; Viswanathan, B.; Renken, A.; Varadarajan,
T. K. Appl. Catal., A 2005, 282, 67-71.
(54) Wojtowicz, H.; Brzaszcz, M.; Kloc, K.; Mlochowski, J. Tetrahedron 2001,
57, 9743-9748.
(55) Barton, D. G.; Shtein, M.; Wilson, R. D.; Soled, S. L.; Iglesia, E. J. Phys.
Chem. B 1999, 103, 630-640.
(56) Gao, X.; Bare, S. R.; Fierro, J. L. G.; Banares, M. A.; Wachs, I. E. J.
Phys. Chem. B 1998, 102, 5653-5666.
(57) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: New York,
1984.
(58) Bass, J. D.; Solovyov, A.; Pascall, A. J.; Katz, A. J. Am. Chem. Soc. 2006,
128, 3737-3747.
9
1126 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Oxygen Coordination in Grafted Ti−Calixarenes
A R T I C L E S
Figure 4. Position and intensity of the maximum of the pre-edge region
of the XANES spectra for Ti-containing materials and compounds.
Compounds 2b (+) and 4a (×) were analyzed in toluene solution. Powder
materials and pretreatment temperatures in K are as follows: 3a-none (x),
3a-393 (b), 3a-523 (X), 3a-823 (O), 3b-393 (9) 3c-none (1), 3c-393 (2),
and 3c-823 (4). Data for 3a-l and 3a-u are close to those of 4a and are
not plotted here for clarity. Rectangles show the region for each Ti
coordination in Ti oxides and silicates, as taken from Farges et al.61
Figure 3. Ti K-edge XANES spectra of (A) all analyzed samples as
synthesized and (B) details of XANES spectra evolution on 3a as prepared,
in air (-b-); in an Ar atmosphere after dehydration in Ar, 2 h at 393 K
(s); and in an Ar atmosphere after dehydration in Ar, 2 h at 523 K (-O-).
The inset amplifies the corresponding pre-edge region.
of treatment temperature; they are typical of those for all
materials based on calix[4]arene-Ti complexes 2a and 2b. As
shown previously for dispersed surface56 and bulk frame-
work10,25,26 Ti centers in SiO2 structures, pre-edge intensities
for 3a increased after treatment at 393 K in Ar, because the Ti
coordination number decreases upon removal of water adsorbed
during exposure to ambient air. Higher treatment temperatures
(up to 523 K) did not cause any additional changes in pre-edge
intensity, indicating that, as in the case of TS-1,10 materials based
on 2a and 2b are essentially free of adsorbed water at 393 K.
Thermal treatments (up to 523 K) did not increase the X-ray
absorption pre-edge intensity of 3c, suggesting that its Ti centers
are unaffected by atmospheric moisture and resistant to changes
in ligand structure in this temperature range.
Phenomenological relations between the coordination sym-
metry of Ti atoms in Ti oxides60 and silicates61 and the position
and intensity of the pre-edge feature in Ti K-edge spectra have
been reported. Energies and intensities characteristic of 4-, 5-,
and 6-fold coordination are shown in Figure 4 for natural
minerals and for the catalysts and compounds used here. These
data show that catalysts and compounds based on calix[4]arene-
Ti complexes 2a and 2b give peak energies characteristic of
[4]Ti (4968.6 eV - 4968.8 eV), but relative peak heights more
typical of [5]Ti or [6]Ti (0.2-0.5). If these materials contained
pure Ti oxides or silicates, these energies and intensities could
be interpreted as evidence for a mixture of [4]Ti and [6]Ti
centers.60,61 Lower intensities and energies than expected for
[4]Ti, however, can also reflect a higher occupancy of 3d states
than for [4]Ti sites within Ti oxides60 or silicates as a result of
non-oxide ligands and of the resulting distortion from tetrahedral
symmetry. Such distortions prevail in other catalytically relevant
Ti-SiO2 materials and are expected also for materials based
on 2a.
0.80)63 exhibit intense pre-edge features similar to those in [4]Ti
centers within natural minerals. Surface-grafted [4]Ti in anhy-
drous TivMCM41 (height ) 0.55)6 and Ti-silsesquioxane
monomers (height ) 0.60)63, however, are distorted from
tetrahedral symmetry by the Ti(OSi)3Y ligand field, in spite of
their 4-fold coordination. Similarly, CpxTiCl(4-x) (x ) 0-2)
compounds exhibit similar near-tetrahedral symmetry for all
values of x, but their pre-edge intensities differ because of the
varying covalent character of their Ti-Cl bonds.23 All these
compounds have pre-edge intensities significantly lower than
those for the [4]Ti region shown in Figure 4.
Although TivMCM41 and Cp2TiCl2 provide precedents for
isolated [4]Ti with pre-edge intensities lower than expected from
tetrahedral coordination, we use soluble compounds 2b and 4a
as model compounds to assign an unequivocal Ti coordination
number to the grafted calixarene-Ti active sites in materials
3a and 3b. To assign the coordination number to the experi-
mentally observed XANES spectra of compounds 2b and 4a,
we use the previously determined single-crystal X-ray diffraction
structure of 2b21 and, because 4a has not been successfully
crystallized, the structure of a V4+ analogue64 of 4a. Metal
centers in 2b and 4a are connected to three calixarene phenols
at ∼0.18 nm, in a geometry also found in other calixarene-Ti
complexes.17,19,20,22,65 The remaining phenolic O-atom lies 0.23-
0.24 nm from the Ti atom, significantly farther than oxygen
atoms in the first coordination sphere of fresnoite66 or anatase,67
and even farther than in H2O physisorbed on TiO2(110),68 which
occurs at 0.22 nm and is not generally observed to influence
(62) Davis, R. J.; Liu, Z.; Tabora, J. E.; Wieland, W. S. Catal. Lett. 1995, 34,
101.
(63) Thomas, J. M.; Sankar, G.; Klunduk, M. C.; Attfield, M. P.; Maschmeyer,
T.; Johnson, B. F. G.; Bell, R. G. J. Phys. Chem. B 1999, 103, 8809-
8813.
(64) Hoppe, E.; Limberg, C.; Ziemer, B.; Mugge, C. J. Mol. Catal. A: Chem.
2006, 251, 34-40.
Anhydrous TS-1 with tetrahedral symmetry (pre-edge height
) 0.85)10,62 and homogeneous Ti(OSiPh3)4 (pre-edge height )
(65) Olmstead, M. M.; Sigel, G.; Hope, H.; Xu, X.; Power, P. P. J. Am. Chem.
Soc. 1985, 107, 8087-8091.
(66) Moore, P. B.; Louisnat, Sj. Z. Kristallogr. 1969, 130, 438-448.
(67) Horn, M.; Schwerdt, Cf.; Meagher, E. P. Z. Kristallogr. 1972, 136, 273-
281.
(59) Stewart, S. J.; Fernandez-Garcia, M.; Belver, C.; Mun, S. B.; Requejo, F.
G. J. Phys. Chem. B 2006, 110, 16482-16486.
(60) Waychunas, G. A. Am. Mineral. 1987, 72, 89-101.
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ReV. Lett. 2005, 95, 226104.
(61) Farges, F.; Brown, G. E.; Rehr, J. J. Phys. ReV. B 1997, 56, 1809-1819.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1127

A R T I C L E S
Notestein et al.
pre-edge spectral features. Although the interaction between the
Ti atom and the ether oxygen is detected by shifts in the 13C
NMR spectrum,15 it is expected to be very weak and not to
lead to detectable features in the near-edge or fine structure of
X-ray absorption spectra because of the Ti-O distance and the
thermal and structural disorder expected in both supported and
solution species. For compounds 4a and 2b, the Ph3SiO- or
Cl- ligands make up the fourth coordinating species; we
therefore consider the experimental XANES pre-edge positions
and intensities of these two soluble compounds (4968.8 eV and
0.29-0.35 relative intensity) to reflect the presence of isolated
and nearly 4-coordinate calixarene-Ti.
The XANES pre-edge peak location and intensity for catalysts
based on 2b or 2c and treated above 393 K lie within 0.1 eV
and 0.1 intensity units of those for model compounds 2b and
4a in solution (Figure 4). Taken together with the finding that
epoxidation turnover rates on 3b are similar to those on other
isolated Ti-SiO2 catalysts,15 these data indicate the prevalence
of [4]Ti in these materials. All catalysts based on calix[4]arene,
irrespective of calixarene-Ti surface density, Ti precursor type
(chloride or isopropoxide), or use in epoxidation catalysis give
similar near-edge spectra, which resemble those for the soluble
model compounds. These similarities strongly suggest that the
SiO2 surface acts as a monodentate ligand similar to Ph3SiO-
in 4a, and that the calixarene-Ti connectivity is retained in all
these materials, consistent with their nature and function as
single-site 4-coordinate Ti active centers.
Coordination Environment of Homooxacalix[3]arene Com-
plexes. In contrast with the similar XANES spectra of all
materials and compounds based on calixarene-Ti complexes
2a and 2b, pre-edge features for 3c are much weaker (∼0.1 vs
∼0.35) and occur at higher energies (by >0.4 eV). This indicates
a higher average Ti coordination number than in 3a and 3b
and in compounds 2b and 4a. The spectrum for 3c is unaffected
by treatment temperatures up to 523 K; thus, the retention of
ambient moisture is not responsible for its higher average Ti
coordination. The energy of the pre-edge peak for 3c lies
between those for anatase and for samples with isolated [4]Ti,
as in small TiO2 crystallites,33,39,69,70 TiO2 containing O vacan-
cies59,71 (Figure 2c), or nitrogen-doped TiO2,72 for which oxygen
vacancies or the prevalence of surface Ti atoms leads to average
Ti coordination numbers of ∼5. These analogies lead us to
propose that Ti atoms in 3c are also, on average, 5-coordinate.
We also show below that this 5-fold coordination in 3c arises
from a derivative of the multidentate calixarene 1c on isolated
Ti centers, and not from the formation of small TiO2 clusters.
UV-visible spectra show that calixarene-Ti connectivity
exists in 3c, and solid-state 13C CP/MAS NMR spectra indicate
that isopropoxide groups present in the Ti precursors used are
no longer connected to Ti, consistent with covalent grafting of
Ti-calixarenes onto SiO2. Material 3c has a calixarene surface
density similar to that of catalysts 3a and 3b, corresponding to
saturation coverages for random calixarene deposition. After
calcination, materials 3c-823 and 3a-823 give similar UV-
visible spectra (Figure S2), which, as shown for other calcined
Ti-SiO2 materials,8,10,73,74 suggests a similar Ti dispersion for
these two samples. Most importantly, the relative pre-edge
heights of the XANES spectra of 3a-823 and 3c-823 are
identical within the accuracy of the measurements, confirming
the similar dispersion in these two materials. Moreover, the
relative pre-edge heights of these materials (0.46 for both) are
comparable to other highly dispersed Ti-SiO2 materials such
as TivMCM41 (height ) 0.55)6. From these results, we conclude
that Ti atoms in material 3c are as isolated and uniformly
dispersed as in materials 3b and 3c. This Ti site isolation and
the differences in the XANES spectra for as-synthesed 3a and
3c are both a consequence of the persistent coordination of bulky
multidentate ligands.
We have shown above that calix[4]arene-Ti complexes 2a
and 2b create [4]Ti atoms when grafted on SiO2 surfaces.
Calixarene 1c contains only three phenolic oxygens; thus, any
additional coordination to Ti in 3c would require O-atoms from
the macrocycle, from silica, or from any adsorbed water retained
at 523 K. Compounds 2a and 2c were grafted onto identical
supports; thus, we expect that SiO2 acts as a monodentate ligand
for 3c, as it does for 3a. Water can be stabilized by interactions
with metal centers within cavities in calixarene-metal com-
plexes,22,75,76 but we find no evidence here for such host-guest
complexes. Single-crystal X-ray diffraction structures for 2c77
do not show any Ti atoms within 0.24 nm of the ether oxygens;
thus, such Ti-O interactions, if present, are not expected to
influence the near-edge spectra, as discussed above. We find,
however, carbonyl resonances in the solid-state 13C CP/MAS
NMR spectra of 3c and vibrational bands in its infrared
spectrum, suggesting that salicylaldehyde-type structures (Scheme
1) may be present. Persistent coordination of Ti to three phenols,
the SiO2 surface, and a carbonyl formed from the dibenzyl ethers
in the 1c macrocycle, as discussed in above, appears to account
for the higher than 4-fold Ti coordination in 3c. The spectro-
scopic resemblance of materials 3c and 3a, except for the
additional carbonyl coordination to Ti centers, provides an
excellent molecular system, with well-defined coordination and
complete accessibility, to test the effects of Ti-O coordination
on epoxidation turnover rates.
Calixarene-Ti Catalytic Reactivity. Materials based on 2b
are active and selective catalysts for epoxidation of alkenes by
alkyl hydroperoxides.15 Here, materials based on 2a and 2c were
used for epoxidation of cyclohexene and 2,5-dimethyl-3-hexene
using TBHP in n-octane at 333 K to probe the catalytic
consequences of calixarene ligands and Ti coordination on
epoxidation catalysis. Cyclohexene epoxidation on 3a reached
50% conversion of TBHP in ∼2 h, and continued to nearly
complete TBHP conversions (>95%). More than 90% of the
TBHP converted is used in epoxidation reactions and no
oxidation products (e.g., cyclohexene-one, cyclohexene-ol,
cyclohexane-diol, or C8 alcohols) except cyclohexene epoxide
were detected, indicating that TBHP losses reflect only adsorp-
tion onto SiO2 surfaces. Epoxide ring-opening was avoided by
maintaining strictly anhydrous conditions,78,79 and to a lesser
(73) Hutter, R.; Mallat, T.; Baiker, A. J. Catal. 1995, 153, 177-189.
(74) Fraile, J. M.; Garcia, J. I.; Mayoral, J. A.; Vispe, E. J. Catal. 2005, 233,
90-99.
(75) Mongrain, P.; Douville, J.; Gagnon, J.; Drouin, M.; Decken, A.; Fortin,
D.; Harvey, P. D. Can. J. Chem. 2004, 82, 1452-1461.
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2556.
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Synchrotron Radiat. 1999, 6, 445-447.
(70) Fernandez-Garcia, M.; Martinez-Arias, A.; Hanson, J. C.; Rodriguez, J.
A. Chem. ReV. 2004, 104, 4063-4104.
(71) Belver, C.; Bellod, R.; Stewart, S. J.; Requejo, F. G.; Fernandez-Garcia,
M. Appl. Catal., B 2006, 65, 309-314.
(77) Hampton, P. D.; Daitch, C. E.; Alam, T. M.; Pruss, E. A. Inorg. Chem.
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9
1128 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Oxygen Coordination in Grafted Ti−Calixarenes
A R T I C L E S
Table 1. XANES Pre-edge Features and Epoxidation Rate Constants Per Ti for Experimental Materials
pre-edgea
2
b
1
calix
−
Ti nm-
pretreatment T (K)
position (eV)
height (I/I0)
(AA2
+
AA3)/AT
epoxidation ratec k1 (M-2 s-
)
2b
4a
3a
3a
3a
3a
3a
3a-u
3a-l
3b
3c
3c
3c
3c
3c
anatase
n/a
n/a
n/a
n/a
4968.8
4968.8
4969.0
4968.7
4968.4
-
4968.9
4968.9
4968.8
4968.6
4969.6
4969.3
g
0.35
0.29
0.20
0.35
0.40
-
0.83
0.77
0.78
0.83
0.86
0.1d
0.4
7.9
6.9
5.8
5.7
4.6
0.22
0.22
0.22
0.22
0.22
0.22
0.13
0.24
0.18
0.18
0.18
0.18
0.18
n/a
nonee
393
523
573
823f
393
393
393
nonee
393
523
573
823f
nonee
-
0.46
0.30
0.26
0.44
0.11
0.13
0.11
-
0.78
0.77
g
-
-
0.89
0.54
0.58
g
-
0.71
11.1d
0.4
0.5
0.7
1.7
3.6
-
-
4968.9
0.45
a
b
(0.1 eV, (0.03 I/I0.
Here, the area weighted average of the peak locations of features A2 and A3 agrees with the maximum of the total pre-edge
c
feature. Relative areas are given as the area of peaks A2 + A3 versus the total area of all pre-edge peaks (AT) (0.07. Epoxidation rate constant k1 per Ti
as described in Experimental Methods, M-2 s-1
.
From Notestein et al.15
XANES samples were measured as received, catalysis samples treated under
d
e
f
g
dynamic vacuum 1 h at ambient temperature. Calcined in flowing N2/O2. Very low S/N, pre-edge feature cannot be accurately located or decomposed
into individual Gaussians.
extent by the weak binding of ethers by Ti-calixarene
complexes.19,21 At the conditions used, contributions from radical
oxidation processes are negligible.15,80 Catalysts are compared
here based on their rate constant k1 (as given in eq 1; per Ti)
for epoxide formation over the first 2 h of reaction (Table 1).
As discussed previously, calix[4]arene-based catalysts such as
3b are stable and do not leach active species or Ti, and their
cyclohexene epoxidation rates are similar to those on other
isolated Ti-SiO2 materials when cumene hydroperoxide is used
as the oxidant and at higher concentrations than used in this
study.15 The supernatant liquid phase in catalytic reaction
affected only weakly by the additional presence and type of
calixarene ligands. The pronounced sensitivity of these site-
isolated grafted Ti catalysts to the steric bulk at the double bond
requires that reaction of the alkene with the activated hydrop-
eroxide occurs at or before the rate-limiting step in the
epoxidation reaction.
In the absence of an organic ligand, the similar surface
densities and accessibility of randomly distributed Ti centers
in materials 3a and 3c suggest that these materials would show
similar catalytic turnover rates. Instead, epoxidation rate con-
stants per Ti for 3a are up to 20 times greater than for 3c after
thermal treatments below 523 K (Table 1). Rate constants for
3a decreased monotonically (by ∼30%) as the treatment
temperature increased to 573 K. This decrease in reactivity
occurred concurrently with a slight decrease in coordination
number detectable in the near-edge spectrum (see Table 1); it
may reflect the involvement of surface SiOH sites in kinetically
relevant epoxidation elementary steps27,82-86 and a concurrent
decrease in their total number with increasing treatment tem-
perature.46 For 3c, treatments below 523 K did not influence
k1, but higher temperatures (573-623 K) led to a sharp increase
in epoxidation rate constants, concurrent with the decomposition
of 3c, detected by thermogravimetry (Figure S3). Complete
oxidative removal of calixarene ligands for 3a-823 and 3c-823
led to a further decrease in k1 for 3a and an additional increase
for 3c. After this oxidative treatment, the catalytic rate constants
(per Ti) of 3a-823 and 3c-823 differ by <25%, providing final
evidence that Ti atoms in both materials are dispersed on the
SiO2 support to similar extents. That is, once the overriding
influence of the organic calixarene ligand is removed, the
catalytic reactivity and the XANES spectra discussed above are
indicative of well-dispersed Ti centers on SiO2, without any
evidence for the formation of extended crystallites of TiO2.
mixtures of material 3c is inactive for epoxidation, and the 13
C
CP/MAS NMR and diffuse reflectance UV-visible spectra are
not significantly changed in intensity after catalysis, proving
the absence of leaching in these materials as well.
The relative epoxidation rates of trans- versus cis-2,5-
dimethyl-3-hexene (normalized by their respective concentra-
tions) were used to probe the relative accessibility of Ti centers
in each catalyst. Alkene epoxidation rates are known to increase
with increasingly electron-rich double bonds;1 thus, trans/cis
isomers are expected to have the same intrinsic reactivity, but
any spatial constraints would restrict access preferentially for
the bulkier trans-isomers. This rate ratio is ∼0.19 for both 3a-
393 and 3c-393, indicating that Ti centers are similarly
accessible in both catalysts.81 Removal of the calixarene ligand
to form 3a-823 increases only slightly the trans/cis rate ratio
(0.22); thus, calixarene ligands do not impede access to Ti
centers grafted onto their lower rim. The stoichiometric oxidant
m-chloroperbenzoic acid gives a rate ratio of 1.2 in noncatalytic
epoxidation. Therefore, Ti centers grafted onto SiO2 surfaces
are spatially constrained relative to epoxidation of molecules
in homogeneous media, but the severity of these constraints is
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become inactive.
9
J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007 1129

A R T I C L E S
Notestein et al.
Figure 5. Representative fits of the pre-edge region of XANES spectra
after treatment in Ar at 393 K for samples (a) 3a-u, (b) 3c, (c) 3b, and (d)
anatase ([6]Ti standard) measured at ambient temperature in air. For all
spectra, the pre-edge region is resolved into four Gaussian peaks corre-
sponding to different electronic transitions and labeled in order of ascending
energy A1, A2, A3, and B.40 An extra peak is fit to empirically account for
edge rising. Fluorescence intensities are normalized by their average values
between 5050 and 5200 eV.
Figure 6. Position and relative area of peaks A2 and A3 for Ti-containing
materials and compounds. Compounds 2b (+) and 4a (×) were analyzed
in toluene solution. Powder materials and pretreatment temperatures in K
are as follows: 3a-none (x), 3a-393 (b), 3a-523 (X), 3a-823 (O), 3b-393
(9) 3c-none (1), 3c-393 (2), and 3c-823 (4). Reference Ti oxides and
silicates (*) are as indicated in the figure. Error bars are estimated from
confidence of fitting procedures. The peak position of A2 + A3 is an area-
weighted average and the sum of their areas are normalized with respect to
the total pre-edge peak area (AT ) AA1 + AA2 + AA3 + AB).
Comparison of Relative Pre-edge Areas and Epoxidation
Rates. Materials based on 2a and 2c differ in epoxidation rates
and in how ligand removal influences their reactivity. These
materials were prepared by similar methods and exhibit identical
UV-visible LMCT energies, steric constraints, and maximum
Ti surface densities. After calixarene ligand removal, these
materials exhibit similar turnover rates, XANES and UV-visible
spectra. As a result, these materials provide excellent models
for comparing the effects of Ti electron density, measured by
XANES, on reactivity for molecular rearrangements catalyzed
by Lewis acids.
Figure 4 shows average Ti coordination numbers in Ti oxides
and silicates, as typically inferred from Ti K-edge XANES pre-
edge peaks using methods first reported by Farges et al.61 and
Waychunas.60 As in a recent study,59 we deconvolute the relative
intensities of the individual electronic transitions giving rise to
the pre-edge features, as determined by Grunes,40 as an improved
method for assessing Ti coordination, especially for distorted
environments prevalent specifically in 5-coordinated systems
and more generally in highly dispersed materials. Specifically,
we use the energy positions of transitions A2 and A3 (See Figure
5 for fits of representative materials) and their combined area
relative to that of all pre-edge features as a measure of Ti 3d
occupancy. As in the case of the overall pre-edge feature
intensity and location, these relative peak areas can be related
empirically to structures of known Ti coordination number, as
in Figure 6. From this figure, it is immediately evident that
materials 3a and 3b have approximately 4-coordinate structures,
as predicted, and that the plot position of 3c is consistent with
5-coordinate Ti centers. All compounds and catalysts derived
from 2a and 2b show a higher relative intensity for peaks A2
and A3 than catalyst 3c. Also, peaks A2 + A3 for catalysts based
on 2a and 2b appear more than 0.4 eV below those for 3c.
These differences in peak position and intensity persist after
all thermal treatments below 523 K (Table 1). This relative peak
area provides a measure of the extent of Ti 3d orbital availability
in these materials, an electronic feature of materials that is likely
to correlate to binding and reactivity of molecules much more
Figure 7. Epoxidation rate constants per Ti as a function of the relative
area of peaks A2 + A3 for Ti-containing materials 3a-393 (b), 3a-823 (O),
3b-393 (9), 3c-393 (2), and 3c-823 (4). Error bars are estimated from
confidence of fitting procedures. Use of the A2 + A3 relative peak area
rather than the total pre-edge feature height predicts a monotonic relationship
between catalyst reactivity and Ti K-edge XANES for all calixarene-
containing and calcined Ti-SiO2 materials.
directly than a geometric description, such as coordination
number.
We therefore use the relative A2 + A3 peak area as a
continuous measure of Ti 3d occupancy and relate it here
directly to epoxidation reactivity. Figure 7 shows cyclohexene
epoxidation rate constants (k1, per Ti, TBHP co-reactant) as a
function of the relative intensity of peaks A2 + A3 for catalysts
3a, 3b, and 3c pretreated at 393 K and after calcination. The
almost linear correlation between relative peak intensities and
the logarithm of k1 suggests that epoxidation activation energies
are indeed related to pre-edge features of Ti centers. This
correlation persists over 20-fold changes in rates (∼8 kJ/mol
in apparent activation energy). Moreover, even though materials
3a and 3b and both calcined materials would all be considered
4-coordinate, the overall correlation persists for small differences
in rates and near-edge features. For example, the decrease in
9
1130 J. AM. CHEM. SOC. VOL. 129, NO. 5, 2007

Oxygen Coordination in Grafted Ti−Calixarenes
A R T I C L E S
rate of 3a after calcination is accompanied by a small decrease
in the relative A2 + A3 area. Using the relative pre-edge feature
height for the ordinate does not give such a monotonic
correlation when calcined and calixarene-containing catalysts
are compared together, indicating more general applicability
when the relative A2 + A3 area is used. The similar linear-free
energy relationship found for calixarene-containing and calcined
materials indicates a similar epoxidation mechanism for both
types of samples. A similar correlation between epoxidation
yield with TBHP and XANES pre-edge features was developed
for sol-gel TiO2-SiO2 materials,87 further indicating the general
nature of the trends shown in Figure 7.
In contrast to the trend identified above, compounds 2b and
4a give spectra nearly identical to those of 3a, but their
epoxidation rates are much smaller. The soluble catalysts must
therefore be operating via a different mechanism than the
heterogeneous catalysts or suffer from a rapid deactivation
pathway (e.g., formation of unreactive dimer species) absent in
immobilized materials of identical composition. These conclu-
sions underscore the need to compare structurally and mecha-
nistically similar catalysts, provided by the series of grafted
calixarene-Ti materials described here, to reach relevant
inferences and structure-function relations. The data in Figure
7 provide a rigorous correlation between d-orbital occupancy,
Ti-O coordination, and epoxidation reactivity, which appears
to be general for calixarene-containing and calcined materials,
while also confirming that the structure of the grafted Ti-
calixarene detected by X-ray absorption is preserved during
catalytic epoxidation cycles. Furthermore, these data demonstrate
that the presence of an additional donor ligand has a pronounced
effect on epoxidation rates when comparing catalysts 3a and
3c. This is in contrast to predictions based on density functional
theory that the epoxidation activation energy for oxygen transfer
from TiOOR intermediates would be insensitive to the presence
of an additional monodendate ligand.88,89 Our results therefore
suggest either a stronger metal-alkylhydroperoxide interaction
than previously proposed or a strong decrease in the epoxidation
pre-exponential factor when an additional coordinated ligand
is part of a multidentate complex.
metal species with known single-crystal X-ray diffraction
structures. In addition to the previously proven single-site
catalytic behavior for these well-dispersed Ti-containing materi-
als, these materials are also site-isolated Ti-SiO2 catalysts.
Moreover, the similarity of the XANES spectra regardless of
calixarene-Ti surface density, use as an oxidation catalyst, or
the identity of the non-calixarene ligand proves that the
macrocyclic calixarene ligand acts as a template for the ultimate
Ti coordination on the surface.
In contrast to the similar catalytic activity and structure of
materials based on 2a and 2b, materials based on homoxacalix-
[3]arene-Ti 2c possess an increased average Ti coordination
number. This coordination number is persistent to 523 K,
eliminating the possibility of simple coordination sphere expan-
sion by a physisorbed species. The resulting catalyst 3c also
possesses a reactivity for epoxidation decreased by as much as
20 times in comparison to that of 3a, indicating persistent
coordination of the calixarene macrocycle during catalytic
turnover. After ligand removal, 3c-823 is similar to material
2a-823 in both catalytic reactivity, UV-visible spectroscopy,
and XANES, indicating that the degree of Ti isolation is similar
in each case, and therefore that the differences in coordination
number are solely due to the overriding influence of the different
calixarene ligand geometries.
These calixarene-Ti-SiO2 materials are used to develop a
quantitative correlation between the occupancy of the Ti 3d
orbital, as judged by the relative area of peaks A2 + A3 in the
XANES pre-edge, and the rate constant for the epoxidation of
cyclohexene using TBHP. A monotonic decrease in apparent
activation energy over ∼8 kJ/mol accompanies the increase in
the relative A2 + A3 peak area for calixarene-containing and
calcined catalysts. This result illustrates the utility of Ti XANES
in quantitatively predicting Lewis-acid catalysis activity and
proves that these calixarenes can be used to create new surface
structures that are relevant during catalytic turnover and can be
directly compared to solid oxide surfaces.
Acknowledgment. We acknowledge the financial support of
the U.S. DOE Office of Basic Energy Sciences (DE-FG02-
05ER15696). The technical assistance of Dr. Aditya Bhan with
infrared measurements, the assistance of Dr. Andrew Solovyov
and Dr. Namal DeSilva with solution 1H NMR characterization,
and the assistance of Dr. Sonjong Hwang at the Caltech Solid-
State NMR facility are greatly appreciated. We also acknowl-
edge ANPCYT (PICT 06-17492), Argentina; CONICET (PIP
6075), Argentina; CONICET-CNPq-NSF collaborative research
agreement (CIAM program), and LNLS, Campinas, Brazil
(project D04B-XAFS1-3492). V.I.K. acknowledges the technical
assistance of Dr. A. Drapailo for the synthesis of the homo-
oxacalix[3]arene 3a and financial support from the Science &
Technology Center in Ukraine Grant 3655. J.M.N. acknowledges
the National Science Foundation for a graduate fellowship.
Conclusion
Titanium present in low concentrations in silicate frameworks
as 4-coordinate species are known to be active for Lewis acid-
catalyzed reactions, whereas 6-coordinate TiO2 domains are
generally inactive; less is known about intermediate geometries.
The synthesis of new Ti-SiO2 materials based on calixarene-
Ti precursors provides access to 4-coordinate Ti using calix-
[4]arene-Ti complexes 2a and 2b, and higher coordination
numbers using homoxacalix[3]arene-Ti 2c. Diffuse reflectance
UV-visible and solid-state NMR spectroscopy show that intact
calixarene-Ti complexes are covalently grafted to the surface
at densities corresponding to the maximum random packing of
the calixarene ligand, ensuring site isolation of grafted Ti centers.
Ti K-edge XANES spectra of all dried materials based on 2a
or 2b indicate [4]Ti with increased Ti 3d occupancy based on
comparison to known Ti-SiO2 catalysts and soluble calixarene-
Supporting Information Available: Characterization of com-
pound 4a, alternate perspectives for the proposed structure of
3c (Scheme S1), FTIR spectrum of 3a, 1c, 3c, and 3c after
exposure to tert-butyl hydroperoxide (Figure S1), UV-visible
spectra of 3a, 3c, and 4a under different conditions (Figure S2),
and TGA of 3a and 3c (Figure S3). This material is available
free of charge via the Internet at http://pubs.acs.org.
(87) Imamura, S.; Nakai, T.; Kanai, H.; Shiono, T.; Utano, K. Catal. Lett. 1996,
39, 79-82.
(88) Yudanov, I. V.; Gisdakis, P.; Di, Valentin, C.; Rosch, N. Eur. J. Inorg.
Chem. 1999, 12, 2135-2145.
(89) Sever, R. R.; Root, T. W. J. Phys. Chem. B 2003, 107, 4090-4099.
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